Blender 2.79 Windows 64Bit - Web-Cam starts on Start-up
I found the issue. It's really somewhat amusing:

I got and installed a new Graphics-Card on my system. (Geforce GTX 970)
The set-up also installed NVIDIA GeForce Experience (V. 3.10.0.95) and during install it set-up also "Game internal overlay", which basically opens the possibility for gamers to record videos of their game, the gamer himself, via WebCam and voices/sound.

The strange behavior is, that the Nvidia software recognizes Blender as a "Game" and switches on all the stuff. (I am not a gamer, so I never realized, that this is available at all)

After setting off this feature, everything turns back to normal.